handle div by zero edge case in maxRedeem

[trmid]

Adds a check in the maxRedeem function to return zero if the prize vault is in a lossy state, but there are absolutely no assets left. This is an extremely unlikely scenario since it requires the yield vault to have lost all funds, but at least the hypothetical prize vault depositors can be soothed by the knowledge that the prize vault is still 4626 compliant after a yield vault's unfortunate demise.

[linear]

GEN-1777 [136] handle maxRedeem div by zero edge case

[github-actions]

LCOV of commit a812f89 during coverage #576

Summary coverage rate:
  lines......: 99.6% (237 of 238 lines)
  functions..: 100.0% (65 of 65 functions)
  branches...: no data found

Files changed coverage rate:
                              |Lines       |Functions  |Branches    
  Filename                    |Rate     Num|Rate    Num|Rate     Num
  ==================================================================
  src/PrizeVault.sol          |99.5%    200| 100%    51|    -      0